A widely-tunable substrate-integrated balun filter

A novel differential coupling structure for tunable evanescent-mode cavity resonators is presented in this paper. The coupling structure is very simple and compact, and presents no design or fabrication challenges relative to a comparable single-ended coupling structure. This new coupling structure is used to realize a high-performance 3-pole tunable balun bandpass filter, which integrates the functionalities of a tunable bandpass filter and a balanced-to-unbalanced transformer (balun). The filter tunes from 3.2 to 6.1 GHz, and has a nominally 2.4% 3-dB fractional bandwidth. It demonstrates state-of-the-art measured amplitude and phase balance among tunable balun filters, with less than 0.2 dB and 0.9
^o
of in-band amplitude and phase imbalance across its entire tuning range.